BEIJING (REUTERS) - North Korea is seeking a peace treaty with the United States, China and South Korea to formally end the Korean War and will not stop its nuclear tests until it gets one, a person who relayed that message from North Korea to China told Reuters.

North Korea announced on Wednesday it carried out its fourth nuclear test since 2006, drawing threats of more sanctions, although the US and weapons experts doubted Pyongyang's claim that the device was a hydrogen bomb.

The test has again raised questions among world powers about what can be done to stop the North's nuclear weapons programme.

The source, who has contacts in the North Korean capital of Pyongyang and correctly predicted the North's first nuclear test in 2006, said the tests would go on until the North's demand for a treaty was met.

"North Korea will do it to the end until China and the United States want to sign a peace treaty," said the source, who declined to be identified.

The 1950-53 Korean War ended in an armistice, not a peace treaty, signed by the US - representing UN forces - the North Korean military and the Chinese army.

Now North Korea wants those three sides and South Korea to sign a treaty.

"This explosion is mainly for the United States to see. The main objective is to persuade the United States to enter into four-country negotiations to end the war so that there can be everlasting peace on the Korean peninsula," the source said.

North Korea has repeatedly said it wants a peace treaty to formally end the war, which it says will give it the security it needs, given what it sees as a hostile US intent on"regime change" in Pyongyang.

The US and China have both dangled the prospect of better ties, including the lifting of sanctions and eventually a likely peace treaty, if North Korea gives up its nuclear weapons.

But North Korea believes the US will only negotiate if Pyongyang can demonstrate its strength through its weapons. With its demand for a treaty ignored, North Korea has continued to develop its nuclear weapons and a stalemate has ensued.

The source said he had relayed the message from North Korea to China's top leadership immediately after its latest test, urging China to support a push for a treaty.

"China should not follow the United States," the source said, referring to the US demand that North Korea give up its nuclear programme before any negotiations.

"Not mentioning a peace treaty is a strategic mistake."

China is the reclusive North's main economic and diplomatic backer, and the two fought side-by-side during the Korean War.

Despite their old friendship, China opposes the North's nuclear programme and has supported sweeping UN sanctions on North Korea for its weapons development.

But China has been reluctant to take tougher action, such as completely shutting their shared border, fearful North Korea could collapse in chaos.

Chinese Foreign Ministry spokesman Hua Chunying, asked on Friday what China's position was on the need for a peace treaty, said China supported resolving all parties "reasonable concerns"and achieving lasting peace through the so-called six party talks.

In 2005, North Korea reached an agreement with the US, South Korea, China, Japan and Russia to suspend its nuclear programme in return for diplomatic rewards and energy assistance.

Negotiations collapsed after the last round of talks in 2008, with North Korea declaring the deal void after refusing inspections to verify compliance.

Pressure is growing on China to do more.

US Secretary of State John Kerry said on Thursday he had made clear in a phone call with Chinese Foreign Minister Wang Yi that China's approach to North Korea has not succeeded.

"Today, in my conversation with the Chinese, I made it very clear that has not worked and we cannot continue business as usual," Kerry said.

Chinese spokesman Hua said in response that the origin of the nuclear issue did not lie with China.

"The key to resolving the issue is also not on China," she said.

The source with contacts in Pyongyang said North Korea was already largely cut off from the world after decades of sanctions, and more would not work.

"North Korea is used to sanctions and not afraid," the source said, adding that the latest test pointed to advances in its weapons.

"The (TNT) equivalent of this explosion was not big, pollution was very small. This demonstrates the technological level is high. It is even easier to make it bigger," the source said.

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